Conventionally, an image intensifier (I.I) is used for photographing an X-ray image, which is carried out in order to diagnose a digestive system including the stomach and the intestine, and the heart.
The image intensifier (I.I) is useful in a medical sight because a moving image obtained through the image intensifier is very effective. That is to say, if still images of internal organs always working are merely photographed, the obtained images are not ones that a doctor expects to obtain in some cases. Therefore, it is necessary to carry out the diagnosis using their moving images. In addition, since it is necessary that timing of photographing of the still image is judged while looking at the moving image, effectiveness of the moving image is very high in the medical care site.
The image intensifier (I.I), as shown in FIG. 7, includes a fluorescent screen 101 which is obtained by depositing cesium iodide (CsI) onto a glass surface by an evaporator, a cathode plate 102 disposed so as to face the fluorescent screen 101, an electron lens portion (anode plate) 103 for condensing electrons emitted from the cathode plate 102 to accelerate the condensed electrons, and an output surface 104 for converting an image of the electrons condensed by the electron lens portion 103 into a visible image to display the resultant image. Note that the output surface 104 is formed by depositing a phosphor onto an aluminum film by an evaporator.
X-rays 110 which are emitted from an X-ray source 111 to be transmitted through the human body 109 are first converted into an X-ray image by the fluorescent screen 101 obtained by depositing cesium iodide (CsI) onto the glass surface by the evaporator.
The X-ray image from the fluorescent screen (CsI) 101 is converted into an electron image by the cathode plate 102 facing the fluorescent screen 101.
The electron image is condensed and accelerated by the electron lens portion 103 to be applied to the output surface (fluorescent surface) 104 to thereby be converted into the visible image by the output surface (fluorescent surface) 104.
The image displayed on the output surface (fluorescent surface) 104 also can be displayed on a monitor 107 through observation with a TV camera 105 or a CCD camera.
However, such an image intensifier (I.I), in principle, has the following problems.
A first problem is such that since the electron lens is used, the image is distorted. A second problem is such that since there is a limit to a size of the electron lens and the cathode surface, a large field of view can not be obtained. A third problem is such that since the apparatus is of large scale, it is difficult to handle the apparatus in a small X-ray room.
On the other hand, in recent years, an X-ray image pickup device using a flat panel detector (hereinafter referred to as “an FPD” for short) has been made fit for practical use along with progress in the semiconductor technology, and is expected to be developed in the future.
Advantages of an X-ray image pickup device using the FPD are such that this X-ray image pickup device has sensitivity and image quality superior to those of an X-ray image pickup device using a film, management of an image becomes simple due to digitization of an image, a new diagnosis method based on an image processing can be established, and so forth. In addition to those advantages, the X-ray image pickup device using the FPD has such an excellent advantage as to be able to photograph a moving image as well as a still image.
Thus, if the FPD can be applied to an X-ray moving image, it is possible to realize an X-ray moving image photography apparatus with which there is obtained an image less in distortion than that obtained through the photographing with the image intensifier (I.I), and also there is obtained a field of view identical to that of a film having a large square size. Moreover, since the apparatus can be thinned as compared with the image intensifier (I.I) and no high voltage is required, it is possible to realize an X-ray moving image photography apparatus which is easy to handle.
In such a manner, by adopting the X-ray moving image photography apparatus using the FPD, it is possible to solve the problems such as distortion of an image which the image intensifier (I.I) has. In addition, since a still image and a moving image can be photographed with one apparatus, efficiency of X-ray image analysis can be increased, and also a load applied to a patient can be reduced. From these points, the moving image photographing using the FPD receives attention.
In the FPD, as shown in FIG. 8, X-rays 220 are applied from an X-ray source 219 to the human body 221 on the basis of an input manipulation using an X-ray source console 215 by an operator. Then, the X-rays transmitted through the human body 221 are converted into visible rays by a phosphor 201. An image based on the resultant visible rays 202 is read at the same magnification by a sensor substrate 203 which is obtained by forming amorphous silicon on a glass substrate through an amorphous silicon process.
The sensor substrate 203 is such that a plurality of pixels each including a photosensor and a switching element for ON and OFF for an output signal from the photosensor are two-dimensionally disposed. An X-ray image read by the sensor substrate 203 is outputted in the form of an electrical signal.
Moreover, after the outputted electrical signal is amplified by a signal amplification circuit 204, the electrical signal is then sent to a control substrate 224 through a relay substrate 223 to be converted into a digital signal by an analog-to-digital converter (A/D converter) 206 provided in the control substrate 224. In addition, a computer 208 for control provided in the control substrate 224 carries out the control so as to supply an electric power of a power source 207 given from an external power source 214 to the relay substrate 223, and also output a control signal to the relay substrate 223.
The relay substrate 223 can transfer the control signal outputted from the control substrate 224 to the signal amplification circuit 204, and also can form a power source required for the sensor substrate 203, a vertical drive circuit 205, and the signal amplification circuit 204.
Image data obtained as the digital signal through the A/D conversion is processed into a moving image by an image processing device 209 to be displayed on a monitor 218. All operations of an X-ray image photography apparatus are controlled by a control PC 211 having the image processing device 209, a program/control board 210, and the like disposed therein.
In addition to the above-mentioned operations, synchronization with the X-ray source 219, storage of an image, printing of an image, connection to an intra-hospital network, and the like can be carried out in accordance with the control made by the control PC 211.
Note that in the foregoing, the image data is stored in a memory device 222, an external memory device 217 or the like.
In addition, the control PC 211 is operated on the basis of an input manipulation using the sensor console 213 by an operator.
One pixel of the above-mentioned FPD is shown in FIG. 9. One pixel is constituted by a metal-insulator-semiconductor (MIS) type photosensor, and a thin film transistor (TFT) disposed as a switching element.
The pixel is formed on a glass substrate 308.
More specifically, the TFT includes a gate electrode 301 made of chromium or aluminum, an insulating film 302 formed of an amorphous silicon nitride film, a channel layer 303 made of amorphous silicon hydride, an N+-type amorphous silicon layer 304 for providing ohmic contact between the channel layer 303 and a metal electrode, and a source electrode 305 and a drain electrode 306 each made of metal such as chromium or aluminum.
In addition, the MIS type photosensor is a MIS type amorphous silicon photosensor and includes a sensor lower electrode 309 made of metal such as chromium or aluminum, an insulating layer 310, as an insulating layer of the MIS type photosensor, formed of a silicon nitride film, a photoelectric conversion layer (I-type layer) 311 made of amorphous silicon hydride, an N+-type amorphous silicon layer 312 for providing ohmic contact between the photoelectric conversion layer 311 and an electrode and for blocking holes generated in the photoelectric conversion layer 311, and a sensor bias line 313 which is made of aluminum, chromium or a transparent electrode material such as indium tin oxide (ITO) and which serves to supply a voltage to the MIS type photosensor.
Moreover, a protective layer 317 for protecting the MIS type photosensor and the TFT from humidity and a foreign matter, a phosphor 315 for converting radiation into visible rays, an adhesion layer 316 for adhesion between the phosphor 315 and the protective layer 317, and a phosphor protective layer 314 for protecting the phosphor 315 from humidity are formed above the TFT and the MIS type photosensor. Also, in the pixel shown in FIG. 9, a signal line 307 is connected to the drain electrode 306.
An amorphous silicon process is used during formation of the FPD because a film having a large area can be uniformly deposited to allow the characteristics of the detector to be unified.
A principle of an operation of the MIS type photosensor will hereinafter be described with reference to energy band diagrams of the MIS type photosensor shown in FIGS. 10A to 10C.
FIG. 10A shows a state during an operation for accumulation (photoelectric conversion mode) in the MIS type photosensor.
When a positive voltage is applied to a side of the sensor bias line 313 of the MIS type photosensor, holes 403 generated within the photoelectric conversion layer 311 due to the photoelectric effect move to an interface between the insulating layer 310 and the photoelectric conversion layer 311 (photoelectric conversion layer-insulating layer interface), while electrons 402 move to a side of the N+-type amorphous silicon layer 312.
At this time, the holes 403 can not move to the lower electrode layer 309 side because they can not penetrate through the insulating layer 310. As a result, the holes 403 are accumulated in the photoelectric conversion layer-insulating layer interface. Thus, a voltage proportional to the amount of irradiation of light 401 and a time period of irradiation of the light 401 is generated in the MIS type photosensor.
However, if a certain amount of holes 403 are accumulated, as shown in FIG. 10B, the voltage due to the holes 403 accumulated in the photoelectric conversion layer-insulating layer interface become equal to the voltage applied to the MIS type photosensor. As a result, an electric field substantially becomes absent in the photoelectric conversion layer 311.
Under this state, the holes 403 generated in the photoelectric conversion layer 311 can not move to the photoelectric conversion layer-insulating layer interface and hence disappear. As a result, the voltage proportional to the amount of irradiation of the light 401 and a time period of irradiation of the light 401 becomes substantially absent. This state is called a saturated state.
In order to provide a state in which the voltage proportional to the amount of irradiation of the light 401 and a time period of irradiation of the light 401 is generated again for the MIS type photosensor held in the saturated state, as shown in FIG. 10C, the voltage on the sensor bias 313 has to be made lower than that in each of the states shown in FIGS. 10A and 10B to sweep out the holes accumulated in the photoelectric conversion layer-insulating layer interface. This operation is called a refresh operation.
Thus, in order that the MIS type photosensor may output the output proportional to the amount of irradiation of the light 401 and a time period of the irradiation of the light 401, it is necessary to repeatedly carry out a series of operations including the accumulation operation, the light irradiation operation, the signal reading operation, and the refresh operation (accumulation operation→light irradiation→signal reading→refresh operation).
However, for realization of the moving image photographing image pickup device using the MIS type photosensor as described above, the refresh operation becomes a problem.
This reason is as follows. That is, if all the pixels are simultaneously refreshed, the moving image will be photographed with a cycle in which the signal reading operation (accumulation operation) and the refresh operation are successively carried out (signal reading (accumulation operation)→refresh operation). However, since information in the refresh operation is not photographed, the moving image becomes unnatural.
In addition, in a case where the refresh operation is carried out whenever the reading operation is carried out several times, if the amount of irradiation of the X-rays differs depending on positions, a certain position is in the saturated state. As a result, there is encountered a problem that no gradation of the image can be obtained, and hence the moving image becomes unnatural.